1. Field of the Invention
The present invention relates to an exhaust gas treatment method and system for a cement burning facility using a chlorine bypass to inhibit a possible increase in the concentration of chlorine in the system of the cement burning facility.
2. Description of the Related Art
In recent years, in order to solve waste disposal problems, various wastes are used as part of a cement material or part of a heating fuel used in a cement kiln. However, when wastes such as a synthetic resin are fed into the cement kiln as part of the fuel, a volatile chlorine component is generated during combustion. The chlorine component is entrained in exhaust gas discharged from the cement kiln and fed to an upper stage side of a preheater. However, when the atmosphere temperature decreases to at most the melting point of the chlorine component as the exhaust gas is transferred to the upper stage side of the preheater, the chlorine component is condensed and adheres to the cement material. The chlorine component is then fed again to the inside of the cement kiln. Furthermore, the chlorine component is evaporated again in response to an increase in atmosphere temperature.
Thus, the chlorine component drawn into the system of the cement burning facility is circulated through the cement kiln and the preheater while being repeatedly evaporated and condensed. Furthermore, a chlorine component generated from newly fed wastes is added to the circulating chlorine component. This increases the concentration of the chlorine component to cause, for example, coating, resulting in blockage in the preheater. Thus, disadvantageously, the facility is prevented from operating stably, and the quality of manufactured cement clinker is also affected.
Thus, in order to solve the above-described problem, for example, Japanese Patent No. 3318714 proposes a kiln exhaust gas treatment method based on the chlorine bypass, which method including a step of extracting part of kiln exhaust gas from a kiln, a step of cooling the extracted exhaust gas to at most the melting point of a chlorine compound, a step of using a classifier to separate dust in the exhaust gas into coarse powder and fine powder, and a step of returning the separated coarse powder to the kiln and feeding the fine powder to the lower stage side of the classifier, wherein the rate of extraction amount of the kiln exhaust gas is set to more than 0% and at most 5%, the separation particle size in the classifier is set to 5 μm to 7 μm, and the feed amount of the fine powder is set to at most 0.1% of the production amount of the clinker.
According to the kiln exhaust gas treatment method configured as described above, the exhaust gas containing the fine powder dust separated by the classifier and having a high chlorine content is fed to a dust collector. Furthermore, the fine powder dust with the high chlorine concentration is collected and discharged to the outside of the system. Thus, the rotary kiln can be stably operated. Additionally, the chlorine can be effectively removed with a minimized heat loss and with the required amount of extracted gas reduced. This reduces the required size of the treatment facility. As a result, both the required space and cost of the facility are reduced, allowing the kiln to be economically and stably operated.
However, in the conventional kiln exhaust gas treatment method described above, the particle size of the fine powder dust collected by the dust collector is set to within the range of very small particle sizes, for example, between 5 μm to at most 7 μm. This prevents the use of a general-purpose classifier, and a classifier exhibiting high performance needs to be installed. Furthermore, the classifier requires precise control during operation. This disadvantageously increases the installation cost of the classifier.
In addition, the very fine powder dust is collected, extremely increasing the concentration of chlorine in the collected fine powder dust. Thus, if the chlorine concentration of the fine powder dust exceeds 20%, then for example, deliquescence of the chlorine compound adsorbed to the fine powder dust may significantly degrade the handling property of the collected fine powder dust. Then, disadvantageously, the fine powder dust adheres to the inner peripheral wall of a conveying pipe, a hopper, or the like (to cause coating), resulting in the likelihood of an accident such as clogging of the hopper or a chute. This unfortunately hinders stable operation and increases maintenance costs.
Moreover, if organic sludge such as sewage sludge is introduced, as the above-described wastes, into a kiln inlet part or a calciner of the cement kiln for treatment, metal chloride (KCl and NaCl) contained in the exhaust gas as a chloride reacts with moisture contained in the organic sludge and CO2 and SO2 in the exhaust gas as shown by the following expression. The metal chloride thus becomes sulfate or carbonate and generates hydrogen chloride (HCl).2KCl+SO2+(½)H2O→K2SO4+2HCl2KCl+CO2+H2O→K2CO3+2HCl
Even when cooled close to the room temperature, most of the hydrogen chloride thus taken into the exhaust gas is present in the form of vapor.
Thus, some of the conventional kiln exhaust gas treatment methods can collect the chlorine entrained in the form of metal oxide but not the hydrogen chloride. This may disadvantageously reduce the chlorine bypass amount (the amount of chlorine removed per unit extracted gas amount) in the chlorine bypass. Additionally, hydrogen chloride (HCl) disadvantageously remains in the exhaust gas, which can thus not directly be discharged to the atmosphere. Furthermore, an acid dew point may be reached, resulting in adverse effects such as corrosion of the apparatus.
Moreover, in the above-described cement manufacturing facility, the amount of dust contained in the extracted gas generally varies significantly depending on the operating condition of the cement kiln, the adhesion status of coating, or a change in the number of rotations of the cement kiln.
Thus, the amount of dust collected through the chlorine bypass also varies substantially, thus varying the quality of the collected dust. If the collected dust is finally added to a cement material for cement manufacturing, the quality of the resulting cement may be affected.